A black rectangle drawn in an editor comes off in one click. Here, the area is painted and the image is re-encoded: what was underneath cannot be recovered, because it is no longer there.
What it does not do: Redaction covers what YOU have drawn: nothing is detected automatically, no face is found for you, because that would need a model downloaded from a third party, which this site forbids itself. Check the result before publishing: an area you forgot stays readable. The image is re-encoded, so slightly recompressed; keep the original.
Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.